I only ever did 4 lg once, and always 1 lg for the rest because 4 lg on the show under pressure was a bit woah man lol - here was the puzzle :
100 75 50 25 1 10 @ 254
I actually play Countdown/Scrabble as a hobby in my spare time - especially Countdown - been a loyal servant to the show for years.

I made it to the semi finals, scored ten centuries and lost in the semi finals by 1 pt. I also got the highest ever debut score alongside two others. There were some clips on YouTube but they may have been removed. I have pics from my time on countdown. I'll see if I can find some video. Had a fabulous experience.

Another thought occurred to me.. Jamie Lee Curtis, but then she's done a signing/photo shoot before and said she'd never do another, so that's that theory down the pan. It'd be amazing if it were Harrison Ford.
For me, my bets are Harrison Ford or Arnold Schwarzenegger because to my knowledge, he's never done a photo shoot but turned up to a convention before, like Ford. So my bet is one of those two. Won't be Stallone as he's done a photo shoot before at a convention.